Eddy Current theory is based on the principles of electromagnetism. A coil carrying an AC current is placed on the magnetic test object. This generates circulating eddy currents in the test material close to the surface and these in turn affect the current in the coil by mutual induction. Flaws and material variations affect the strength of the eddy currents. The presence of flaws is therefore measured by electrical changes in the excitation coil. ET applications range from crack detection to metal sorting and weld inspection.